The aircraft industry has long been concerned with the development of safe flight procedures in the presence of inclement weather. Aircraft weather monitoring instrumentation involves a variety of devices including radar- and ground-derived weather information readouts, as well as instruments performing in specific response to thunderstorm phenomena. These latter monitoring instruments are desirable inasmuch as, although radar maps rain with great accuracy, rain is not the pilot's main concern. Hazardous convective activity occurs long before any rain has fallen. Thus, it is helpful to identify thunderstorms where radar indicates nothing.
Thunderstorms have long been studied and evaluated by investigators. Some categorizations of them have been advanced as part of an analysis leading to the detection and mapping instrumentation. For example, in U.S. Pat. No. 4,023,408 by Ryan et al., issued May 17, 1977, and entitled "Stormscope", thunderstorms are classified into two basic types, the convective thunderstorm and the frontal thunderstorm. The discourse provided therein describes the development through maturity of each type of storm. In general, the storms exhibit very fast discharge phenomena which, when monitored, provides an extensive amount of data developed from strikes of a duration on the order of 100 microseconds. Thus, to fully evaluate the storms, it is important that a broad-band response be made to them. Generally, a storm-generated lightning bolt will be present as an extremely large current surge from cloud to ground. Accompanying that surge of current is a magnetic field which propagates as any radio wave for many, many hundreds of miles. With each such electromagnetic wave, there are two components, a magnetic component or H-field, and an electrostatic component or E-field. The E-field is non-directional as far as antenna evaluation thereof is concerned and thus, its influence on any antenna structure serves to contaminate directional information. Typically, the instruments heretofore developed employ a technology wherein three antennas are utilized, two being H-field antennas and a third being a E-field antenna. The latter E-field antenna has represented an unfortunate major source of noise and confusion to the circuitry of these storm monitors, inasmuch as it responds quite well to noise phenomena and thus is quite unreliable. This latter third antenna is used to resolve ambiguity which results from recourse by instruments of the past to low bandwidth performance. The use of this third antenna results, more than likely, because of its use typically in automatic direction finding devices. Such devices have typically employed a pair of cross coils and a third E-field antenna to resolve ambiguities. However, in the automatic direction finding environment, the signal monitored is continuous, whereas when this system is employed in ranging and locating thunderstorms, the signal relied upon is that emanating from the lightning strike, and thus is a mere random pulse of generally known polarity. Thus, the third, E-field antenna conventionally employed with the thunderstorm evaluating and monitoring instrumentation has represented a problematic aspect of those devices.
Conventional storm monitoring instruments have employed two orthogonally-disposed H-field coils which are formed as wire windings about a ferrite core. Such wound structures may pose difficulties in costs in assembly. This core-and-coil assemblage is then mounted within an antenna pod on the aircraft such that one coil is oriented perpendicularly with respect to the longitudinal or flight-heading axis of the aircraft. Storm-bearing information thus is developed in conjunction with aircraft bearing or heading. However, the perpendicular orientation of one of the H-field antennas with respect to heading results in storm-bearing response characteristics which are the least accurate for the aircraft heading direction. Further, the attitude of the aircraft typically evokes a vertical movement or pivoting of the antenna coils during conventional flight. This results in signal anomalies. Another aspect of this conventional H-field antenna implementation concerns its output, which is a function of the derivative of the magnetic field. Thus, integration approaches are employed which are undesirable in such measurement systems.
The aircraft environment also poses problems for storm monitoring instrumentation. Thus, testing of such equipment before flight or the installation of the equipment generally cannot evaluate the effect of the dynamic and noisy environment of an aircraft in flight. During such flight, cabling carrying antenna-derived signals will be subjected to the noise influence of aircraft heater motors, strobe light power supplies, autopilot motors and the like. This, of course, represents a highly noisy and rigorous environment to derogate the value of the signals transmitted. Installation anomalies also may occur. The antenna may be installed backwards or upside down by unskilled technicians or mechanics. Wiring to the H-field antennas may be reversed to generate just the opposite of information desired. Finally, the readout components should be of practical size and weight for instrument panel mounting. Typically, relatively long cathode ray tube (CRT)-based readout devices are employed, requiring specialized mounting due to their inherently larger bulk.